1. Field of the Invention
The present invention relates to leads on surface mount components and more particularly to improvements in the configuration and structure of the terminal part of leads on components designed for solder attachment to a substrate.
2. Description of the Prior Art
Assembly of electrical circuits often involves the mounting of components on carriers or cards by attachment of the terminal part of leads to form assembled printed wiring boards. As used herein, the term "lead" means a device on the component which is designed to provide a connection between the component and a bond site. The "terminal" is that part of the lead designed for direct solder connection to the bond site. For example, L-gullwings, J-type lead terminals, Butt-lead type lead terminals, and terminals on 2-terminal components are all examples of leads, as are the contact pads or terminals on so-called "leadless surface mount components." A multi-lead component, such as a quad, may have hundreds of leads whereas a 2-terminal component would have 2.
Surface Mount Technology involves bonding leads of the component on one surface of a printed circuit board on the card, which printed circuit board is patterned to provide the desired electrical connections between different components and sections thereof. Typically, the terminal of each lead is soldered onto a metallic contact pad, land or footprint on the carrier thereby forming a solder joint. Some contact pads are designed so the lead and terminal provide electrical connection between the electrical component and the printed circuit board. Some contact pads might not be designed to provide such an electrical connection. Regardless, the lead and solder joint usually provide mechanical support.
It is general practice for the terminal to have a flat bonding surface (gullwing) or to be J-shaped, and for the corresponding contact pad to be flat. It is well known in the art that a solder joint formed between such a terminal and the contact pad can be weak because of volatiles and bubbles trapped in the solder. Additionally, due to the shape of the solder joint, it may have low resistance to mechanical and thermal shear and stress. One factor contributing to these characteristics is that the terminal has a limited area for adhesion. Furthermore, the solder might migrate or flow to adjacent solder joints during reflow, thereby causing a short-circuit.
The above described characteristics of prior art apparatus and technique may result in a defective board. It is often difficult, costly and time consuming to rework, that is isolate, and remove and replace or correct the defective connection.
U.S. Pat. No. 4,777,564 to Denis J. Derfiny discloses a leadform which provides a more compliant and adhesive solder connection by virtue of an "elf toe" on the terminal. In other words, the terminal presents a convex surface for soldering to the contact pad. The solder can thus adhere to the larger surface area. EP No. 0265631 to IBM discloses a tapered or beveled head on the lead designed to discourage redistribution and migration of the bonding material.
The conventional reflow method for connecting the lead to the contact pad starts with depositing solder paste or cream containing flux and additives on the contact pad, usually by screen or stencil printing. The terminal is then placed on the contact pad. Reflow will heat the solder, the terminal and the contact pad until the solder melts and then forms the bond as it cools.
These known methods are subject to various difficulties. Some difficulties arise from lack of control over the amount of solder at each connection. Insufficient solder would result in open circuits and voids. Excessive solder would cause a physically weak connection. Regardless, there might be bridging and consequential short circuiting between adjacent solder joints. Reflowing is a bulky and expensive technique. Solder paste has a limited shelf life. Furthermore, reworking, that is, removing and replacing a costly component or solder joint, is extremely difficult.
Prior art efforts to solve these problems include predepositing solder on the terminal before bonding the terminal to the contact pad. The following patents are hereby referenced as being typical of known prior art insofar as they disclose predepositing of solder on the terminals before attachment to the carrier:
______________________________________ Patent No. Inventor ______________________________________ 4,722,470 Ajay Johary 4,657,172 Jong S. Lee 4,607,782 Bruce L. Mims 4,396,140 Donald Jaffe et al. ______________________________________
The inventions of Johary and Jaffe have the disadvantages of requiring manual operation and are not suitable for mass production. Furthermore, they involve solder paste. In Lee, solder is applied to inverted J-leads by wave soldering. This invention is limited to J-leads and it has the disadvantages inherent in wave soldering. In Mims, solder adheres to the terminal after the terminal is dipped in a smear of solder paste on a plate. Because there is no specific pad or cavity for each lead, this invention does not adequately control the amount of solder at each connection.
Accordingly, it is a general object of this invention to provide improved Surface Mount component leads so as to improve the reliability in solder connection between the lead and the mounting substrate which overcomes the aforementioned difficulties.
It is a further object of this invention to provide for an improved lead which includes a terminal portion which results in enhanced reliability and fewer defects in solder connections.
It is a further object of this invention to provide an economical method to predeposit solder on the terminal part of the lead in precisely regulated amounts, which method is well suited for automation.
It is a further object of this invention to provide improved Surface Mount component terminals which facilitate rework operations.